Conventional X-ray imaging systems installed in a hospital and the like are classified into a film radiography type in which a patient is irradiated with X-rays and a film is exposed to X-rays having passed through the patient, and an image processing type in which X-rays having passed through a patient are converted into an electrical signal and the signal undergoes a digital image process.
One of image processing systems is a radiographic apparatus which includes a phosphor for converting X-rays into visible light and a photoelectric conversion device for converting visible light into an electrical signal. X-rays having passed through a patient irradiate the phosphor, and internal information of the patient that is converted into visible light by the phosphor is output as an electrical signal from the photoelectric conversion device. The information can be digitized by an A/D converter after conversion into an electrical signal, and the X-ray image information subjected to recording, display, printing, diagnosis, and the like can be processed as a digital value.
Recently, a radiographic apparatus using an amorphous silicon semiconductor thin film for the photoelectric conversion device has come into practical use (see Japanese Patent Laid-Open No. 2002-305687).
FIG. 12 is a plan view showing a conventional photoelectric conversion substrate constituted by using an amorphous silicon semiconductor thin film as the materials of a MIS photoelectric conversion device and switching device. FIG. 12 also illustrates wiring lines which connect these devices. FIG. 13 is a sectional view taken along the line I-I in FIG. 12. In the following description, the MIS photoelectric conversion device will be simply called a photoelectric conversion device for convenience.
Photoelectric conversion devices 101 and switching devices 102 (amorphous silicon TFTs: to be simply referred to as TFTs hereinafter) are formed on a single substrate 103. The lower electrodes of the photoelectric conversion devices share first metal thin film layers 104 with the lower electrodes (gate electrodes) of the TFTs. The upper electrodes of the photoelectric conversion devices share second metal thin film layers 105 with the upper electrodes (source and drain electrodes) of the TFTS. The first and second metal thin film layers also serve as gate driving wiring lines 106 and matrix signal wiring lines 107 within the photoelectric conversion circuit unit. In FIG. 12, the number of pixels is 2×2=4. A hatched portion in FIG. 12 is the light-receiving surface of the photoelectric conversion device. Reference numeral 109 denotes a power supply line which biases the photoelectric conversion device. Reference numeral 110 denotes a contact hole for connecting the photoelectric conversion device and TFT.
With the structure as shown in FIG. 12 in which an amorphous silicon semiconductor is the main material, photoelectric conversion devices, switching devices, gate driving wiring lines, and matrix signal wiring lines can be simultaneously fabricated on the same substrate. This can easily provide a large-area photoelectric conversion circuit unit at low cost.
Device operation of a single photoelectric conversion device will be explained. FIGS. 14A to 14C are energy band diagrams for explaining device operation of the photoelectric conversion device shown in FIGS. 12 and 13. Depending on how voltage is applied to the first and second metal thin film layers 104 and 105, the photoelectric conversion device has two operation modes: a refresh mode and photoelectric conversion mode.
FIGS. 14A and 14B show operations in the refresh mode and photoelectric conversion mode, respectively, and illustrate the states of the layers in FIG. 13 in the direction of film thickness. Reference symbol M1 denotes a lower electrode (G electrode) formed from the first metal thin film layer 104 (e.g., Cr). An amorphous silicon nitride (a-SiNx) layer 111 is an insulating layer which blocks passage of both electrons and holes. The a-SiNx layer 111 must be thick enough not to cause the tunnel effect, and is generally set to 500 Å or more. An amorphous silicon hydride (a-Si:H) layer 112 is a photoelectric conversion semiconductor layer formed from an intrinsic semiconductor layer (i layer) in which no dopant is intentionally doped. An N+ layer 113 is an injection blocking layer against a carrier of one conductivity type that is formed from a non-single crystal semiconductor (e.g., N-type a-Si:H layer) in order to block injection of holes into the a-Si:H layer 112. Reference symbol M2 denotes an upper electrode (D electrode) formed from the second metal thin film layer 105 (e.g., Al).
In FIG. 12, the D electrode does not completely cover the N+ layer. The D electrode and N+ layer are always at the same potential because electrons freely move between the D electrode and the N+ layer. The following description is based on this.
In FIG. 14A showing the refresh mode, a negative potential is applied to the D electrode with respect to the G electrode. Holes (•) in the i layer (a-Si:H) are guided to the D electrode by the electric field. At the same time, electrons (∘) are injected into the i layer. At this time, some holes and some electrons recombine with each other and disappear in the N+ layer or i layer. If this state continues for a sufficiently long time, holes in the i layer are swept from the i layer.
In order to change from this state to the state of FIG. 14B showing the photoelectric conversion mode, a positive potential is applied to the D electrode with respect to the G electrode. Then, electrons in the i layer are instantaneously guided to the D electrode. However, holes are not guided to the i layer because the N+ layer functions as an injection blocking layer. If light enters the i layer in this state, light is absorbed by the i layer to create electron-hole pairs. Electrons are guided to the D electrode by the electric field, whereas holes move in the i layer and reach the interface between the i layer and the a-SiNx insulating layer. At this time, holes cannot move into the a-SiNx insulating layer, and stay in the i layer. Since electrons move to the D electrode and holes move to the interface between the i layer and the insulating layer, a current flows from the G electrode in order to keep electroneutrality in the photoelectric conversion device. The magnitude of current corresponds to electron-hole pairs created upon incidence of light, and is proportional to the quantity of incident light.
After the state in FIG. 14B showing the photoelectric conversion mode is maintained for a predetermined period, the state changes to that of FIG. 14A showing the refresh mode again. Then, holes staying in the i layer are guided to the D electrode, as described above, and at the same time a current corresponding to holes flows. The number of holes corresponds to the total quantity of light applied during the period of the photoelectric conversion mode. At this time, a current corresponding to the number of electrons injected into the i layer also flows, but the number of electrons is almost constant, and the current can be detected by subtracting the number of electrons. That is, the photoelectric conversion device can output the quantity of incident light in real time, and can also detect the total quantity of light applied during a given period.
However, when the period of the photoelectric conversion mode becomes long for whatever reason or the illuminance of incident light is strong, no current may flown, regardless of the incidence of light. This is because many holes stay in the i layer in the photoelectric conversion mode and reduce the electric field within the i layer, and created electrons are not guided to the D electrode and recombine with holes within the i layer, as shown in FIG. 14C. This state is called the saturation state of the photoelectric conversion device, if the incident state of light changes in this state, current may flow, but unstably. However, holes in the i layer are swept in the refresh mode again, and a current proportional to light flows again in the next photoelectric conversion mode.
In X-ray imaging using the conventional radiographic apparatus, the photoelectric conversion device is set in the refresh mode to perform refresh operation. Then, the photoelectric conversion device is set in the photoelectric conversion mode and irradiated with X-rays to perform read operation, acquiring one still image. In order to acquire successive moving images, the series of processes is repeated by the number of moving images to be acquired.
After refresh operation, X-ray irradiation must wait until voltage fluctuations by the refresh stabilize. In general, specifications necessary for imaging of a chest are an imaging region of 40 cm square or more and a pixel pitch of 200 μm or less. When the radiographic apparatus is fabricated at an imaging region of 40 cm square and a pixel pitch of 200 μm, the number of photoelectric conversion devices increases to 4,000,000. Refreshing so many pixels increase the current flowing during refresh operation, and voltage fluctuations in GND and the power supply line of the X-ray imaging apparatus become large. In other words, the wait time after the refresh operation becomes longer for a larger number of pixels.
To acquire moving images by using the conventional radiographic apparatus, refresh operation must be executed after each imaging operation, as described above. As the number of pixels increases or the number of imaging operations increases, the time taken for refresh and the wait time accompanying refresh become longer, decreasing the frame frequency.